Remove Appveyor CI builds

This completely removes all Appveyor CI build jobs to move Alacritty to
a singular CI provider for all platforms.

To ensure that Windows is still properly tested, the allowed failure for
the stable Windows build job on travis has been removed.

This fixes #1679.

.travis.yml

@@ -24,7 +24,6 @@ matrix:
       env: ARCH=i386
   allow_failures:
     - rust: nightly
-    - os: windows
 
 install: ci/install.sh
 script: ci/script.sh

appveyor.yml

@@ -1,64 +0,0 @@
-# Based on the "trust" template v0.1.1
-# https://github.com/japaric/trust/tree/v0.1.1
-
-environment:
-  global:
-    CRATE_NAME: alacritty
-    RUST_BACKTRACE: 1
-    APPVEYOR_SAVE_CACHE_ON_ERROR: true
-
-  matrix:
-    - platform: x86_64
-      TARGET: x86_64-pc-windows-msvc
-      RUST_VERSION: stable
-      CLIPPY: false
-
-    - platform: x86_64
-      TARGET: x86_64-pc-windows-msvc
-      RUST_VERSION: nightly
-      CLIPPY: true
-
-matrix:
-  allow_failures:
-    - CLIPPY: true
-
-shallow_clone: true
-
-install:
-  - curl -sSf -o rustup-init.exe https://win.rustup.rs/
-  - rustup-init.exe -y --default-host %TARGET% --default-toolchain %RUST_VERSION%
-  - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
-  - rustc -Vv
-  - cargo -V
-
-build_script:
-  # Only build once per architecture
-  - if [%CLIPPY%]==[false] (
-      cargo +%RUST_TOOLCHAIN% build --release
-    )
-
-before_test:
-  - If [%CLIPPY%]==[true] (
-      rustup component add clippy-preview
-    )
-
-test_script:
-  - if [%CLIPPY%]==[true] (
-      cargo +%RUST_TOOLCHAIN% clippy
-    )
-    else (
-      cargo +%RUST_TOOLCHAIN% test &
-      copy target\debug\winpty-agent.exe target\debug\deps &
-      cargo +%RUST_TOOLCHAIN% test -p winpty
-    )
-
-cache:
-  # Ideally we'd also cache the toolchain but it's too large to fit in the free cache.
-  # Building alacritty takes significantly longer than downloading the toolchain.
-  - target
-
-artifacts:
-  - path: target\release\alacritty.exe
-    name: Alacritty
-  - path: target\release\winpty-agent.exe
-    name: Winpty agent